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Hot Nests Mean Baked Baby Leatherbacks

By Rachel Nuwer May 23, 2012 5:00 pmMay 23, 2012 5:00 pm

Ruth Fremson/The New York TimesNewly hatched sea turtles make their way to the sea in Costa Rica.

As if sea turtles didn’t already have enough troubles. On beaches, poachers snatch up their eggs and babies for stewing; at sea, adults get snagged by fishermen’s long lines and nets. Now, climate change joins the list, threatening the survival of critically endangered leatherback sea turtles in the Pacific.

James Spotila, the Betz Chair professor of environmental science at Drexel University in Philadelphia, says that in decades to come, global warming is likely to heat up the beach and kill off turtles. “They’re facing not just one problem, but a convergence of many negative effects of both people and climate change,” he said in an interview.

To arrive at this conclusion, Dr. Spotila and colleagues monitored leatherbacks arriving to lay eggs at Las Baulas National Marine Park in Costa Rica for six nesting seasons between 2004 and 2010. To create the next generation, female leatherbacks dig holes on beaches and deposit their eggs in about two and a half feet of sand. In the course of the study, the researchers located 814 nests (representing about 31 percent of all nests) and waited for the babies to emerge, usually around two months later.

Two days after the first hatchlings burst out of the sand, they would dig up the nest to count the number of dead babies and spoiled eggs.

In Costa Rica, weather patterns are governed by the El Niño and La Niña weather cycles. The years in which El Niño reigned, with hot, dry conditions, corresponded to bad seasons for turtles, and the opposite was true for the cooler, wetter La Niña cycles. When conditions are hot and dry, they found, eggs desiccate as the sand bakes in the sun, and the babies are unable to break through the packed, scalding substrate.
The gender of baby turtles also depends upon temperature. Hot conditions produce mostly females, so the babies that did survive the difficult beach conditions were predominantly female, skewing the population mix. Other studies have found these conditions to be equally lethal for olive ridley turtles and green turtles in Costa Rica.

After calculating the hatchling success rate, they overlaid that information with local climate data and created models projecting what could happen in the future, relying on climate variables from the Intergovernmental Panel on Climate Change. Climate variability, they found, will markedly influence the hatching success of leatherback babies. Specifically, El Niño years of low precipitation and high temperatures will detrimentally affect both egg development and the hatchlings’ emergence from the sand, they concluded in a study just published in the journal PLoS ONE.

Under normal conditions, turtles regularly have boom and bust years. Sometimes, an entire reproductive year is lost because of unfavorable weather. But researchers predict that these deadly conditions will only increase as the seasons pass by. They project that hatchling survival in Costa Rica will decline by up to 60 percent between now and 2100 as a result of climate change.

“I’d say this is yet another example in this mounting pile of how global warming and climate change are threatening animals and plants all over the place,” Dr. Spotila said.

Because of strict protection measures at national parks, northwestern Costa Rica remains one of the last major leatherback nesting sites in the region. The country’s beaches annually cradle about 40 percent of all Pacific leatherbacks’ eggs.

The odds of turtles’ locally adapting to climate change are low for a couple of reasons, Dr. Spotila said. First of all, almost all northern beaches that might offer better conditions as things heat up are already covered by high-rise condos and resorts. Costa Rica has put a lot of effort into protecting its turtles. If they moved north to Mexico, for example, they likely would still face the same poaching threats that drove them from those nesting beaches to begin with.

Second, leatherbacks and other sea turtles are extremely long-lived, with some studies estimating they do not reach reproductive age until they are around 30 years old. The few years between now and 2100 represent only a couple generations for the turtles and is just a blink in the evolutionary time scale. This means natural selection would not likely have the opportunity to kick in before climate change takes its toll.

“If we allow the climate to continue to change and heat up, we’re going to lose the turtles in the Pacific — at least certainly in the Eastern Pacific,” Dr. Spotila warned.

As far as nesting populations in areas not affected by El Niño, like Indonesia, separate studies would have to be conducted to predict their fate, he said.

For now, Dr. Spotila sees leatherback survival as a constant struggle. While other researchers fight to alleviate the threats that fisheries and development pose to adult leatherbacks, he grapples with how to make the beach more favorable for nestlings. Although national parks prevent egg poaching, conservationists now face the prospect of having to either install miles of sprinkler units to keep eggs moist, or to devise a shading system to cool hundreds of nests.

Local mitigation, however, will not compensate for a lack of global action in reversing climate trends, the researchers warned.

“I suppose it’s inevitable in a world that’s getting filled up with people that we have to do a lot more to manage any of those species that we want to protect so they can continue to be around,” Dr. Spotila said.

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